EP0236833A2 - Method and apparatus for the automatic analysis and recognition of a telecast encryption code - Google Patents

Abstract

The method consists in evaluating the shift applied to a certain number x of successive lines of the encryption cycle, then in looking up in a table the unique encryption code which, at this position of the cycle, gives the sequence found for the x shift values. In order to analyse the shift in each of these x lines, the method consists in firstly evaluating the relative shift of each of the x-1 lines in relation to the first, by studying the relative shift of a given brightness transition present in all of these x lines and which, before encryption, reflects a detail vertical to the picture, then, secondly, in evaluating the absolute shift of each of these x lines by comparison with a line from the x to which a minimum shift has been applied on encryption.

Description

The present invention relates to the field of so-called encrypted television and relates more precisely to the search for the encryption code, used in the context of television image encryption by applying delays between the image signal of each video line and the line synchronization, these delays being variable and established from one line to another according to a pseudo-random code. Such encryption destroys the vertical structure of the image, making it unintelligible, the lines being offset with respect to each other and, for the same line, from one frame to another of the same parity in the same cycle. Only compensation for these delays in reception makes it possible to restore the synchronization of the various image signals from one line to another, the compensation code itself being a function of the pseudo-random code of delays used during encryption.

Such encryption and decryption devices currently exist in Europe in the context of prepaid televisions, intended to be decoded only by a certain category of subscribers (see French patent 75.34029 and French patent applications 82.05006 and 82.05007).

Swiss patent applications Nos. 87/85 and 1289/85, filed on January 9 and March 22, 1985 by Technical Developments & Investments, describe various methods and devices for analyzing and recognizing the encryption code of television broadcasts , of which the present invention represents only another, faster form of analysis and automatic recognition of the encryption code of television broadcasts.

The object of the invention is to arrive at determining the pseudo-random cycle of the offsets used during encryption, so as to restore it to a suitable decoder which will then be able to provide a correct image from the encrypted version of the image, this without it being necessary for the encrypting body to communicate to each subscriber the new encryption code used at each change thereof.

However, for this invention to be usable, the pseudo-random code of the successive shifts between the line video image signal and the synchronization pulse of this same line, from one line to another, is a cyclic code, that is to say established for a finite number T of frames and reset at the end of these T frames thanks to an indication of cycle synchronization carried by the video broadcast, the audio broadcast or both together (by example, the cycle can be reset every 6 frames thanks to a uniform white level carried by the last line of the end of cycle frame, this last line of frame remaining black in the 5 other frames of the cycle, as suggested by French patent application N 82. 05008).

The number of different possible offsets must also be a finite number; call it d (for example d = 3, i.e. 3 different types of shifts T1, T2, T3 with, for example, T1 = 0, T2 = s, T3 = 2s and with s = 1µs) .

Finally, for certain embodiments of the invention, the number of different possible pseudo-random codes must not be infinite and, in particular, in a determined position of the cycle, there never exists more than n successive values. of common offset for any two different codes taken from the different possible pseudo-random codes.

All these different conditions are effectively fulfilled in the context of current European applications of this encryption mode.

When we speak, within the framework of this patent and in accordance with the technical specificities currently in force, of "successive lines" relative to the code or the encryption cycle, it will always be successive lines of the same frame and not of successive lines on the screen.

The analysis method according to the invention consists in evaluating the relative offset applied to the encryption between two lines, successive or not, by analyzing the position on each of these lines of a determined luminance transition (such a luminance transition is selected by its intensity of white which must be higher than a fixed minimum level). Indeed, insofar as this luminance transition reflects in the image, before encryption, a vertical detail extending at least over the two lines analyzed, the relative offset of this transition between these two lines, after encryption, will be a direct function of the relative difference between the two offsets applied to encryption on these two lines. On the other hand, knowing the relative offset of a line with respect to a line to which an extreme offset value has been applied, makes it possible to deduce the absolute offset applied to this line for encryption. This is notably the case when we know the relative shifts between several lines, each with respect to the previous one or each with respect to the same single line, and when the lines analyzed include at least two lines to which the encryption the two extreme offset values. It then suffices to evaluate, step by step, the relative offset of each of these lines relative to that to which was applied, for example, the minimum offset to deduce therefrom the absolute offset applied to the encryption on each of these lines .

Such an analysis method can notably be used to find the encryption code used for a television program, this by analyzing the relative offset between successive lines two by two until analyzing all of the lines making up the cycle, or by analyzing the relative offset of each of the lines making up the cycle with respect to the same single line, or alternatively by analyzing several groups of lines (the whole of which constitutes the whole cycle) according to one of the methods indicated just before. Among the lines analyzed, there are necessarily at least two lines to which have been applied

two extreme offset values, one of these lines can then be used to define the individual absolute offset of each of the lines constituting the cycle. This set of absolute offsets, classified by successive lines of the cycle, then constitutes the desired encryption code.

The method of analysis and automatic recognition of the encryption code for television broadcasts consists in analyzing the value of the absolute offset of a certain number x (x> n> of successive lines at a determined position of the cycle and in searching, in a comparison table, the unique pseudo-random encryption code which gives, in the chosen analysis position, the x offsets noted during the analysis.

Such a method is notably usable when the number of possible different pseudo-random codes is small, and when the maximum number n of offset values common to two different codes at a determined position of the cycle is small.

The "automatic analysis" part of the method according to the invention consists in evaluating, at a determined position of the cycle, the value of the absolute shifts applied to x successive lines (x> n) by analyzing the relative position, on each of these lines, of a given luminance transition.

Indeed for a sufficiently small value of x (for example 10 lines), it is frequent to find a significant luminance transition which is vertical to the image before encryption, ie whose position is identical on several successive lines before encryption, and which is present on all of the x lines taken into account in the analysis. Such a transition can then be used for analysis since the relative offset of this transition between two lines directly reflects the relative difference between the offsets applied to the encryption between these lines. If, in addition, among the x lines analyzed, there are the two extreme possible offset values, we can deduce the individual offset value applied to the encryption to each of these x lines.

Thus, if at the cycle analysis position chosen, and for any code, we call m the maximum number of successive lines that can be obtained by encryption such that it is possible, among these m lines, not to have the two extreme values of offset, it will suffice to take x> m to deduce from the analysis the individual absolute offsets of the x lines analyzed.

This is why, in order to be able to perform the analysis and the automatic search for the encryption code, x must be taken greater than the greater of the two values of m and n. Likewise, if one wishes to be able to carry out this automatic analysis and search for the encryption code at different points in the cycle, it will suffice to ensure that x is taken greater than the largest of the values of n and m calculated in at least one of the places where this analysis can be carried out.

• -Dans un premier cas de figure, la mémoire dispose de l'ensemble des différentes séries de x valeurs de décalage absolus possibles en une position déterminée du cycle, dite de référence, ainsi que, pour chacune de ces séries, du code de cryptage lui correspondant. Une simple recherche par analogie donnera dans ce cas, pour une analyse de x lignes faite au meme endroit de référence du cycle, le code de cryptage recherché.
• -Dans un second cas de figure l'analyse pourra etre faite en différentes positions du cycle, les x valeurs de décalage trouvées en l'une de ces positions d'analyse et la localisation relative de cette position d'analyse par rapport à la position de référence, suffisant à définir le code de cryptage utilisé. Ce cas de figure est notemment utilisable lorsque les codes de cryptage sont tous des codes pseudo-aléatoires issus d'un meme registre à décalage.

The "automatic recognition" part of the method according to the invention consists in searching in a memory where the necessary information is stored, the unique encryption code which gives the x absolute offset values found at the location of the cycle analyzed.

• -In a first case, the memory has all the different series of x absolute offset values possible at a determined position of the cycle, called the reference, as well as, for each of these series, the encryption code itself corresponding. A simple search by analogy will give in this case, for an analysis of x lines made at the same reference point of the cycle, the encryption code sought.
• -In a second case, the analysis can be done at different positions of the cycle, the x offset values found in one of these analysis positions and the relative location of this analysis position relative to the position sufficient to define the encryption code used. This scenario is notably usable when the encryption codes are all pseudo-random codes originating from the same shift register.

FIG. 2 represents, from the point of view of luminance, in the context of an example of encryption with 3 offset times (0, T, 2T), and for a schematic image signal, all of the 3 video lines ( A, B, C) can be obtained after encryption and, for each of them, the signal from the video comparator (A ', B', C ') for a determined transition threshold R.

FIG. 1 represents an example of an analysis and automatic recognition device of the encryption code implementing the method according to the invention.

• a)Un extracteur de synchronisation 1 permettant d'obtenir à partir de l'image vidéo 100 les impulsions de synchronisation de ligne 2 et de trame 3.
• b)Un extracteur de synchronisation du cycle de cryptage 4, lui-meme fonction du type d'information vidéo de synchronisation de cycle prévue par l'organisme crypteur (dans l'exemple décrit, une information vidéo portée par la dernière ligne de la dernière trame du cycle), et capable de fournir des impulsions de synchronisation de cycle.
• c)Un filtre réjecteur de la sous-porteuse couleur 10.

Such a device includes:

• a) A synchronization extractor 1 making it possible to obtain, from the video image 100, the line 2 and frame 3 synchronization pulses.
• b) An encryption cycle synchronization extractor 4, itself a function of the type of cycle synchronization video information provided by the encryption organization (in the example described, video information carried by the last line of the last cycle frame), and capable of providing cycle synchronization pulses.
• c) A rejector filter of the color subcarrier 10.

It is a "plug" circuit at 4.286 MHz for SECAM; 4.43 MHz for PAL; 3.58 MHz for NTSC. Such a filter makes it possible to filter the chrominance so as to retain only the luminance.

• e)Un circuit comparateur 30 qui délivre un niveau logique 1 lorsque le niveau du signal vidéo est supérieur à un niveau de référence choisi R, permettant ainsi de signaler la présence d'une transition pouvant servir à l'analyse.
• f)Un générateur de fenetre 40 permettant de ne prendre en compte dans l'analyse, et pour chaque ligne, qu'une certaine partie du signal image. Ce générateur de fenetre délivre un niveau logique 1 à l'intérieur des temps à prendre en compte pour l'analyse.

d) A black alignment circuit 20 making it possible to restore the DC component of the signal. This circuit receives the line 2 synchronization pulses.

• e) A comparator circuit 30 which delivers a logic level 1 when the level of the video signal is higher than a chosen reference level R, thus making it possible to signal the presence of a transition that can be used for analysis.
• f) A window generator 40 making it possible to take into account in the analysis, and for each line, only a certain part of the image signal. This window generator delivers a logical level 1 within the times to be taken into account for the analysis.

This window generator receives the line 2 synchronization pulses.

g) An “AND” logic gate 50 which processes the information coming from the window generator on the one hand, and from the comparator circuit on the other hand.

• -évalue le décalage relatif entre deux lignes successives dès qu'existe un niveau logique 1 à la sortie de la porte 50 pour ces deux lignes, et que l'interprétation du décalage relatif entre ces deux lignes, à partir de la position relative de ce niveau logique sur les deux lignes, est compatible avec les décalages de cryptage possibles (c'est, par exemple, le premier des niveaux logiques 1 délivrés pour chaque ligne par le circuit 50 qui est pris en compte pour l'analyse du décalage relatif);
• -évalue le décalage relatif de chacune des lignes suivantes par rapport a la première ligne analysée, ceci tant que la porte 50 délivre un niveau logique 1 sur les lignes suivantes et tant que l'interprétation de la position de ce niveau logique donne, pour ces lignes, un décalage compatible avec les décalages de cryptage possibles, ceci jusqu'à obtenir des valeurs de décalages relatifs portant sur x lignes successives;
• -recommence l'analyse à zéro s'il ne parvient pas à obtenir de niveau logique 1 sur l'une des x lignes successives analysées, ou si la valeur d'un décalage relatif trouvé est incompatible avec les décalages de cryptage possibles, jusqu'à obtenir les valeurs de décalages relatifs portant sur x lignes successives;
• -déduit de l'analyse effectuée sur ces x lignes successives le décalage absolu appliqué au cryptage à chacune de ces x lignes, dès lors que figurent parmis ces x lignes au moins deux lignes auxquelles ont été appliqués les deux décalages extremes au cryptage;
• -recherche dans une table (ROM) le code de cryptage qui donne, en une position choisie de référence, la série de x décalages trouvée;
• -déduit, lorsque la position d'analyse n'est pas la position de référence et que les codes de cryptage sont tous issus d'un meme registre à décalage, de la localisation relative de la position d'analyse par rapport à la position de référence, le véritable code de cryptage qui a été utilisé.
• -fournit à un décodeur adapté 80, une fois le code de cryptage trouvé, les valeurs de décalage de compensation à appliquer au décryptage, chacune successivement au moment du passage de la ligne du cycle lui correspondant.

h) An analysis circuit 60, containing a microprocessor, a RAM and a ROM, which:

• -evaluates the relative offset between two successive lines as soon as there exists a logic level 1 at the exit of gate 50 for these two lines, and that the interpretation of the relative offset between these two lines, from the relative position of this logic level on the two lines, is compatible with the possible encryption offsets (it is, for example, the first of the logic levels 1 delivered for each line by the circuit 50 which is taken into account for the analysis of the relative offset) ;
• -evaluate the relative offset of each of the following lines with respect to the first line analyzed, this as long as the gate 50 delivers a logic level 1 on the following lines and as long as the interpretation of the position of this logic level gives, for these lines, an offset compatible with the possible encryption offsets, until obtaining relative offset values relating to x successive lines;
• - starts the analysis again at zero if it fails to obtain logic level 1 on one of the x successive lines analyzed, or if the value of a relative offset found is incompatible with the possible encryption offsets, up to obtaining the relative offset values relating to x successive lines;
• deduced from the analysis carried out on these x successive lines the absolute offset applied to the encryption to each of these x lines, as soon as these x lines appear among at least two lines to which the two extreme offsets have been applied to the encryption;
• -research in a table (ROM) the encryption code which gives, in a chosen reference position, the series of x offsets found;
• -deduced, when the analysis position is not the reference position and the encryption codes are all from a same shift register, from the relative location of the analysis position with respect to the position of reference, the real encryption code that was used.
• -provides to a suitable decoder 80, once the encryption code has been found, the compensation offset values to be applied to the decryption, each successively at the time of crossing the line of the cycle corresponding to it.

The analysis circuit receives the line 2, frame 3 and cycle 4 synchronization pulses.

i) A switch 70 makes it possible to start the analysis. j) A suitable decoder 80 makes it possible, thanks to the decryption code supplied to it by the analysis circuit from the encryption code found, to transform the encrypted version of the video image 100 into a correctly decrypted version 200.

In order to evaluate the relative offset between two successive lines, the analysis circuit 60 must have an internal clock whose unit of time is at most equal to the largest divider common to the different possible offset times.

Such a device ultimately allows and after a very short analysis time (of the order of a second for a cycle of 6 frames and for images containing luminance transitions appearing vertical to the image before encryption), to recognize the correct encryption code and therefore to give it to a decoder suitable for decrypting television programs thus encrypted.

The security of the encryption device for the encryption organization is fully preserved since only the encryption organization knows all of the different encryption codes that can be used. It therefore remains the only one able to make such automatic code search devices.

The methods and devices according to the invention can also be used for searching for the encryption code when the television programs are not only encrypted as indicated in the preamble to the text of this patent, but also apply a method of reversing the polarity of the video signal (as described in French patent application 75 34029), operated on one or more determined lines of the cycle and for each cycle on the same lines. such polarity reversals can be recognized by a simple circuit, and the signal be restored corrected for this polarity reversal, which makes it possible to apply the methods and device of the present invention, The information indicating that such line of the cycle has undergoing a polarity reversal can easily be kept in a memory, and the information be restored to the suitable decoder so as to rectify this polarity reversal together with the compensation for the shifts it operates.

Finally, to verify that the code found by the automatic analysis and recognition process is the correct one, it is possible to use one of the devices described in the Swiss patent applications filed by Technical Developments § Investments and cited in the introduction to this patent.

Claims (10)

1. Method for analyzing the relative offset between two successive encrypted lines of the same television broadcast frame, the encryption being obtained by imposing a delay, variable from one line to another, between the image signal of the video line and the line synchronization pulse and these delays being established from one line to another according to a pseudo-random code reset cyclically every T frames, characterized in that an analysis circuit searches on these two lines for the presence of a luminance transition greater than a given threshold and evaluates the relative offset of this transition between the two lines, this relative offset being, for a luminance transition reflecting a vertical detail in the image, directly function of the relative difference between the two offsets applied to the two lines analyzed. 2. Method according to claim 1, characterized in that the analysis of the relative offset between two lines can be carried out between two non-successive lines, since the luminance transition which is analyzed reflects a vertical detail in the image. which is present on these two lines, the analysis of the relative offset of this transition between these two lines being directly a function of the relative difference between the two offsets applied to these two lines. 3. Method for analyzing the encryption code according to claims 1 or 2, characterized in that the analysis of the relative offset between two lines is repeated on all the lines constituting the coding cycle, ie in groups of two lines successive, or, if there is a luminance transition reflecting a vertical detail to the image extending over all the lines of the cycle, for each of the lines with respect to a single line of reference, the set of information collected making it possible to deduce ultimately, from the relative offset of each line with respect to two extreme offset lines, the set of individual absolute offsets of each of these lines, which constitutes, in order successive cycle lines, the pseudo-random code used for encryption. 4. Method for analyzing and recognizing the encryption code according to one of claims 1 to 3, characterized in that, when the pseudo-random code used is taken from a finite number of different possible pseudo-random encryption codes and that, on the one hand, there exists in a determined position of the cycle, between any two of these different possible codes, never more than n successive values of offset common for these two codes and that, on the other hand, does not exist in this position of the cycle, never more than m successive lines free from the two extreme possible offset values, it suffices to limit the analysis to x successive lines taken in this determined position of the cycle -x being taken greater than n and m- and to deduce, from the relative position of each of these lines with respect to two extreme offset lines, the value of the absolute offset of each of these lines, a recognition circuit then making it possible to determine, using x values s uccessives of absolute shifts stored at the end of the analysis and which are specific to the encryption code, that of the various pseudo-random encryption codes used. 5. Method according to claim 4, characterized in that when the different possible pseudo-random codes all come from the same shift register, it is possible to carry out the analysis at different places in the cycle as soon as x is chosen greater than the largest of the values of n and m calculated in at least one of the positions of the cycle where the analysis can be carried out, the x absolute offset values obtained for one of the analysis positions, and the location relative of this analysis position with respect to a determined reference position, finally making it possible to recognize the encryption code used. 6. Method according to one of claims 1 to 5, characterized in that the analysis of luminance transition is carried out, for each line, on a given part only of the image signal. 7. Device for implementing the method according to claim 1 or 2, characterized in that it comprises a) a synchronization extractor (1) which makes it possible to extract the line synchronization (2) and frame (3) pulses from the encrypted received television signal (100); b) a circuit for analyzing cycle synchronization information (4), whether video or audio or both, and making it possible to generate cycle synchronization pulses; c) a rejector filter of the color subcarrier (10), making it possible to store video information only the luminance; d) a black alignment circuit (20) making it possible to restore the DC component of the signal; e) a comparator circuit (30) which delivers a logic level 1 when the signal level is higher than a chosen reference level; f) an analysis circuit (60) making it possible to evaluate the offset between two lines taken into account in the analysis, by evaluating the relative variation of the position of a given logic level 1 on these two lines. 8. Device according to claim 7 for implementing the method according to claim 4 or 5, characterized in that the analysis circuit (60) evaluates, for a given position of the cycle, the relative offsets between x successive lines, each with respect to its previous one or each with respect to the first, and deduces therefrom the absolute offset of each of them, then searches in a memory for the unique pseudo-random code which gives, for this analysis position, the sequence x absolute shifts found. 9. Device according to claims 7 and 8 for implementing the method according to claim 6, characterized in that it further comprises a) a window generator (40) making it possible to take into account in the analysis for each line only a determined part of the image signal; b) an "AND" logic gate (50) making it possible to let pass from the comparator circuit only the information located in the window defined by the generator (40). 10. Device according to claim 8, characterized in that the analysis circuit (60) automatically resumes at its start a new analysis of x successive lines as soon as the analysis in progress is not possible on one of the lines analyzed or gives results which are incompatible with possible offsets during encryption.

Images